1. Field of the Invention
This invention relates to the conversion of thermal energy to electric energy, and electric energy to refrigeration, and more particularly to a solid state thermionic converter using semiconductor diode implementation.
2. Relevant Technology
Thermionic energy conversion is a method of converting heat energy directly into electric energy by thermionic emission. In this process, electrons are thermionically emitted from the surface of a metal by heating the metal and imparting sufficient energy to a portion of the electrons to overcome retarding forces at the surface of the metal in order to escape. Unlike most other conventional methods of generating electric energy, thermionic conversion does not require either an intermediate form of energy or a working fluid, other than electric charges, in order to change heat into electricity.
In its most elementary form, a conventional thermionic energy converter consists of one electrode connected to a heat source, a second electrode connected to a heat sink and separated from the first electrode by an intervening space, leads connecting the electrodes to the electric load, and an enclosure. The space in the enclosure is either highly evacuated or filled with a suitable rarefied vapor, such as cesium.
The essential process in a conventional thermionic converter is as follows. The heat source supplies heat at a sufficiently high temperature to one electrode, the emitter, from which electrons are thermionically evaporated into the evacuated or rarefied-vapor-filled interelectrode space. The electrons move through this space toward the other electrode, the collector, which is kept at a low temperature near that of the heat sink. There the electrons condense and return to the hot electrode via external electric leads and an electric load connected between the emitter and the collector.
An embodiment of a conventional thermionic converter 100 is schematically illustrated in FIG. 1. These conventional devices typically comprise an emitter 110, or low electron-work-function cathode, a collector 112, or comparatively colder, high electron-work-function anode, an enclosure 114, suitable electric conductors 116, and an external load 118. Emitter 110 is exposed to heat flow 120 which causes this cathode to emit electrons 122, thus closing the electric circuit and providing an electric intensity to load 118. As indicated above, interelectrode space 130 in conventional thermionic converters is an evacuated medium or a rarified-vapor-filled medium.
The flow of electrons through the electric load is sustained by the temperature difference between the electrodes. Thus, electric work is delivered to the load.
Thermionic energy conversion is based on the concept that a low electron work function cathode in contact with a heat source will emit electrons. These electrons are absorbed by a cold, high work function anode, and they can flow back to the cathode through an external load where they perform useful work. Practical thermionic generators are limited by the work function of available metals or other materials that are used for the cathodes. Another important limitation is the space charge effect. The presence of charged electrons in the space between the cathode and anode will create an extra potential barrier which reduces the thermionic current. These limitations detrimentally affect the maximum current density, and thus present a major problem in developing large-scale thermionic converters.
Conventional thermionic converters are typically classified as vacuum converters or gas-filled converters. Vacuum converters have an evacuated medium between the electrodes. These converters have limited practical applications.
Embodiments in a first class of gas-filled converters are provided with a vaporized substance in the interelectrode space that generates positive ions. This vaporized substance is commonly a vaporized alkali metal such as cesium, potassium and rubidium. Because of the presence of these positive ions, liberated electrons can more easily travel from the emitter to the collector. The emitter temperature in these types of conventional devices is in part determined by the vaporization temperature of the substance that generates the positive ions. Generally, the emitter temperature should be at least 3.5 times the temperature of the reservoir of the positive ion generating substance if efficient production of ions is to be achieved in these conventional devices.
Embodiments in a second class of gas-filled converters are provided with a third electrode to generate ions. The gas in the interelectrode space in these conventional devices is an inert gas such as neon, argon and xenon. Although these converters can operate at lower temperatures, such as about 1500 K, they are more complex.
Typical conventional thermionic emitters are operated at temperatures ranging from 1400 to 2200 K and collectors at temperatures ranging from 500 to 1200 K. Under optimum conditions of operation, overall efficiencies of energy conversion range from 5 to 40%, electric power densities are of the order of 1 to 100 watts/cm2, and current densities are of the order of 5 to 100 A/cm2. In general, the higher the emitter temperature, the higher the efficiency and the power and current densities with designs accounting for radiation losses. The voltage at which the power is delivered from one unit of a typical converter is 0.3 to 1.2 volts, i.e., about the same as that of an ordinary electrolytic cell. Thermionic systems with a high power rating frequently consist of many thermionic converter units connected electrically in series. Each thermionic converter unit is typically rated at 10 to 500 watts.
The high-temperature attributes of thermionic converters are advantageous for certain applications, but they are restrictive for others. This is because the required emitter temperatures are generally beyond the practical capability of many conventional heat sources. In contrast, typical thermoelectric converters are operable at heat source temperatures ranging from 500 to 1500 K. However, even under optimum conditions, overall efficiencies of thermoelectric energy converters only range from 3 to 10%, electric power densities are normally less than a few watts/cm2, and current densities are of the order of 1 to 100 A/cm2.
From a physics standpoint, thermoelectric devices are similar to thermionic devices. In both cases a temperature gradient is placed upon a metal or semiconductor, and both cases are based upon the concept that electron motion is electricity. However, the electron motion also carries energy. A forced current transports energy for both thermionic and thermoelectric devices. The main difference between thermoelectric and thermionic devices is in the transport mechanism: ballistic and diffusive transport for thermionics and ohmic transport for thermoelectrics. Ohmic flow is microscopically diffusive, but not macroscopically so. The distinguishing feature is whether excess carriers are present. In thermoelectrics, the carriers normally present are responsible for current. In thermionics, the current is due to putting excess carriers in the gap. A thermionic device has a relatively high efficiency if the electrons ballistically go over and across the gap. For a thermionic device all of the kinetic energy is carried from one electrode to the other. The motion of electrons in a thermoelectric device is quasi-equilibrium and ohmic, and can be described in terms of a Seebeck coefficient, which is an equilibrium parameter.
In structures with narrow barriers, the electrons will not travel far enough to suffer collisions as they cross the barrier. Under these circumstances, the ballistic version of thermionic emission theory is a more accurate representation of the current transport. The current density is given by:       j    =                  A        0            ⁢              T        2            ⁢              ⅇ                                            -              e                        ⁢                          xe2x80x83                        ⁢            ϕ                                              k              B                        ⁢            T                                ,
where A0 is the Richardson""s constant, xcfx86 is the barrier height (electron work function), e is the electron charge, kB is Boltzmann""s constant, and T is the temperature. Richardson""s constant A0 is given by A0=(emkB2T2)/(2xcfx802h2), where m is the effective electron mass and h is reduced Plank""s constant.
The foregoing electron current density equation provides quantitative language for explaining some of the observations described above. For example, this equation for the emission current shows that the rate of emission increases rapidly with temperature, and it decreases exponentially with the work function.
Solutions to the foregoing problems have been sought according to the present state of the art by using vacuum converters or gas-filled converters. Attempts to reduce space-charge effects with vacuum converters have involved the reduction of the interelectrode separation to the order of micrometers. Attempts to reduce the same effects with gas-filled converters have led to the introduction of positive ions into the cloud of electrons in front of the emitter. Nevertheless, these conventional devices still present shortcomings such as those related to limited maximum current densities and temperature regimes. Consequently, there remains a need to provide a more satisfactory solution to converting thermal energy to electric energy at lower temperature regimes with high efficiencies and high power densities.
The present invention was developed to fill a need for a device which efficiently converts thermal energy to electric energy at relatively low operating temperatures and with power densities and efficiencies high enough for commercial applications. The present invention also operates in reverse mode to provide efficient cooling.
The present invention seeks to resolve a number of the problems which have been experienced in the background art, as identified above. More specifically, the apparatus and method of this invention constitute an important advance in the art of thermionic power conversion, as evidenced by the following objects and advantages realized by the invention over the background art.
For example, one object of the present invention is to generate high power densities and efficiencies of a typical thermionic converter, but to operate at temperature regimes of typical thermoelectric devices.
A further object of the present invention is to perform energy conversion through thermionic emission using semiconductor thermal diode implementation.
Additional objects and advantages of the invention will be apparent from the description which follows, or may be learned by the practice of the invention.
Briefly summarized, the foregoing and other objects are achieved by a solid state thermionic converter utilizing semiconductor diode implementation comprising an emitter that preferably comprises an n*-type region; a gap region between the emitter and a collector, the gap region being preferably adjacent to said n*-type region; and a cold ohmic contact connected to said gap region, said cold ohmic contact having a recombination collector region formed between said cold ohmic contact and said gap region. In some embodiments of this invention a collector provides a recombination element and such collector is in electric communication with a cold ohmic contact. Preferably, a hot ohmic contact is in electric communication with the emitter. The gap region maybe n-type, p-type, or intrinsic. To use the electric current generated by embodiments of the converter of this invention, the electric circuit is typically closed externally with an electric load, preferably connected to the hot ohmic contact and the cold ohmic contact. It is understood that terms such as xe2x80x9celectric communicationxe2x80x9d, xe2x80x9celectric connectionxe2x80x9d and xe2x80x9celectric contactxe2x80x9d refer to a relationship between elements whereby electric current can flow between such elements, whether such elements are in direct contact or the electric current flow is facilitated by at least one conductor linking such elements.
In the refrigeration embodiment, carrier transport is assisted by an external electric field. A first ohmic contact on the emitter, preferably comprising an n*-type region, is connected to a thermal load that is cooled by heat flow from electrons leaving the emitter. As described in connection with preferred embodiments of heat-to-electricity converters according to this invention, electrons in refrigeration embodiments preferably circulate from the emitter, preferably from a hot ohmic contact on the preferably n*-type region, to the gap region. A gap region is preferably adjacent to the emitter, and a second ohmic contact having a recombination collector region is preferably formed between the second ohmic contact and the gap region. The gap region in embodiments of this invention may be n-type, p-type, or intrinsic. A heat exchanger dissipates the heat from hot electrons on the second ohmic contact connected to the gap region.